Conventionally known methods of joining a noble metal tip to a ground electrode of a spark plug are disclosed in, for example, PCT Application Laid-Open No. 2004-517459 and US Patent Application Publication No. 2007/0103046.
According to the method disclosed in PCT Application Laid-Open No. 2004-517459, a noble metal tip is completely melted and joined to a ground electrode. This method can increase the welding strength between the ground electrode and the noble metal tip, but involves a problem of a deterioration in spark endurance, since the discharge surface of the noble metal tip contains components of a ground electrode base metal as a result of fusion.
Also, according to the method disclosed in US Patent Application Publication No. 2007/0103046, a peripheral portion of a noble metal tip is melted, thereby joining the noble metal tip to a ground electrode. This method, however, involves the following problem: the welding strength between the ground electrode and a central portion of the noble metal tip is weak, and cracking may be generated in the noble metal tip or a fusion zone, potentially resulting in separation of the noble metal tip.
Also, a method which uses resistance welding is known for joining a noble metal tip to a ground electrode. This method, however, involves the following problem: since the layer of a fusion zone at the interface between the ground electrode and the noble metal tip is thin, welding strength fails to cope with severe operating conditions, such as with respect to a spark plug that is increased in temperature because of the recent tendency toward higher engine outputs. Such operating conditions can potentially result in separation of the noble metal tip.